A. Field of the Invention
This invention uses multiple observers to passively determine range and bearing to an RF emitter. In particular, it employs emitter wavefront phase change measured over at least two receiver dwells at a single aircraft and pulse time of arrival measurements made between two platforms to perform the geolocation--i.e., the determination of the emitter's latitude, longitude, and altitude. The only observer required to be moving is the aircraft making the phase change measurements.
B. Description of Related Art
Two methods to locate an RF emitter by generating lines-of-position (LOP) on which the emitter must lie are (1) to utilize time-difference of arrival (TDOA) hyperbola, with the TDOA measured between observers, or (2) to associate bearing differences with unique circles as curves passing through both the observer and the emitter. Multiplatform location techniques based on TDOA were thoroughly analyzed by Wegner in "On the Accuracy Analysis of Airborne Techniques for Passively Locating Electromagnetic Emitters" (RAND Report R-722-PR, 1971).
The association of circular lines-of-position with horizontal bearing difference measurements is a technique well known in navigation (see, for example, G. P. Clark, "Simplified Determination of the Ellipse of Uncertainty," Navigation. Journal of the Institute of Navigation, vol. 21, No. 4, 1974). Royal, in U.S. Pat. No. 3,922,533, describes its use in multiplatform RF emitter location.
The use of TDOA for precision location on aircraft flying certain common missions has severe limitations. FIG. 1 provides scenarios that illustrate this. FIG. 1a shows a desirable engagement geometry in which two ground attack aircraft 100 and 101 five miles apart approach emitter 103. The aircraft are spaced close enough together to allow visual contact, which is operationally highly desirable. A TDOA methodology does not support this desirable geometry, as shown by Wegner, since TDOA requires three observers able to simultaneously detect the emitter. Also the TDOA location is most accurate when the three aircraft have the emitter at the center of an equilateral observer triangle, and performance can degrade rapidly for other geometries. For example, if the aircraft are only able to approach the emitter from one direction, which is a common constraint, then obtaining a location accuracy on the order of 125 m circular error probable (CEP) requires the addition of a second flight of two aircraft 104 and 105, as shown in FIG. 1b.
These additional aircraft must be spaced sufficiently far from the initial pair to allow geometry to dominate system errors when intersecting TDOA hyperbola to locate the emitter. But with the 30 nm spacing between observers required to approach a 125 m CEP, the simultaneous detection of the same emitter RF pulse becomes increasingly more difficult to achieve. One aircraft may detect the emitter's mainbeam, while the remaining aircraft fail to simultaneously detect the emitter's sidelobe. And even if multiplatform detection is successful, the problem of data correlation remains. With widely spaced platforms it may be difficult to associate pulses with the same emitter, let alone guarantee the same pulse is used by all observers to measure TOA.
Also, since the emitter location is often initially not even approximately known, it is difficult to optimize placement of the observers. But performance is very sensitive to observer-emitter relative bearings. The following example illustrates this. TDOA performance was derived for both the head-on approach with the emitter at 103, and for the 45.degree. approach shown there with the emitter at 106. The system errors assumed in generating performance included the following: time-of-arrival (TOA) variation due to pulse rise time, video bandwidth, and signal strength effects, where the strength variation arises from signal propagation length differences and receiver measurement degradation; aircraft location errors due to GPS measurement uncertainty; and lack of time synchronization caused by phase error between the reference clock on each aircraft. The system errors assumed produced a TOA error at each observer with a one sigma statistical variation of 34.7 nsec. Geometry-induced errors, or GDOP (geometrical dilution of precision) arise from the shallowness of the angles at which the TDOA-generated hyperbola intercept at the emitter, and the GDOP degradation limits the system error that can be tolerated in any given observer-emitter geometry.
For the head-on geometry of FIG. 1b, the interaction of this TOA error and the GDOP error produced a CEP of 165 m. But for the 45.degree. geometry of FIG. 1b, i.e., with the emitter at 106 rather than 103, the error is over 257 m. This sensitivity of performance to changes in emitter-observer geometry clearly represents an important tactical problem in the use of TDOA to precisely locate emitters.
In contrast to TDOA, multiplatform emitter location with circles-of-position uses only differential measurements made by single observers rather than measurements between observers. Hence no simultaneous multiplatform coordination is required to generate the LOP. But the bearing difference measurements must be made with sufficient accuracy to produce precise enough COPs to locate the emitter to the required circular error probable. The errors can be reduced by increasing the bearing spread 510 in FIG. 5, which means increasing the flight path length 501 and hence total time over which the difference is formed. But this is not a robust approach since the emitter may be on only very briefly.
The applicant's U.S. Pat. No. 5,526,001 presented a method to generate the circles employing phase difference measurements from an uncalibrated two antenna long baseline interferometer (LBI). When measuring 510, .beta., with an interferometer, measurement precision is proportional to interferometer baseline length. Hence use of the LBI, which can have a baseline of several hundred inches, provides resolution that makes it practical in many operational situations to rapidly measure bearing differences against higher frequency emitters and hence do multiplatform geolocation in tactically important situations. But there are still critical limitations, as will be seen.
Since the circles are generated from the LBI phase measurements, they are called phase-circles. FIG. 2 illustrates the use of phase-circles to locate the emitter in the scenario introduced by FIG. 1a. The aircraft 100 and 101 fly short baselegs 205 and 206 for 8 seconds to generate bearing changes at the emitter, and associate the circular lines of position 200 and 201 with these bearing changes. In order to produce the performance shown by 400 in FIG. 4, the bearing change was measured by a 240 inch LBI. The emitter is located by the intersection of the two COP at point 202. The accuracy of the phase-circles is not affected by emitter pulse characteristics, such as pulse width or rise time, as are the TOA measurements. However the accuracy is a function of emitter frequency. This dependence arises from the use of the phase interferometer to make the bearing change measurements. When the emitter is practicing ECCM by not staying on for any appreciable length of time, the accuracy of the bearing change measurement at lower frequencies cannot be reliably improved by flying a longer baseleg then the 8 second leg assumed in generating the FIG. 4 results.
Hence the low frequency emitter location accuracy of the phase-circle method is not satisfactory in some important cases, and in particular for the scenario shown in FIG. 1a. As the FIG. 4 performance curve 400 demonstrates, the goal of 125 m CEP can only be achieved at 6 GHz and above for the 8 second measurement interval allowed.
Insert 207 in FIG. 2 indicates the problem. The GDOP for the phase-circles is severe in the FIG. 1a scenario due to the shallow intersection angle of the COP. The lines 204 and 205 give the COP perturbation due to 3.degree. 1s phase measurement errors. These measurement errors are due to navigation system (NAV) attitude errors in spatially locating the LBI baseline, antenna vibration induced errors, antenna phase mistrack bias, receiver calibration phase bias, thermal noise and quantization errors. It is extremely difficult to reduce the errors below the .+-.30.degree. assumed. These errors produce the 1s emitter location region 206. The area of this region is clearly very sensitive to COP perturbations. A small increase in COP uncertainty due to phase measurement error, e.g. downward shift in COP inner bound 203, induces a substantial degradation to emitter location accuracy.
This invention increases the system error that can be tolerated for a given observer-emitter geometry. It also decreases the sensitivity of location accuracy to changes in that geometry. In particular it reduces the frequency limitation aspects of the phase-circle approach by linking it with TDOA. The link is done in a way that also largely alleviates the shortcomings in measuring multiplatform TDOA since the TDOA is measured between closely spaced observers., e.g. 100 and 102 in FIG. 1a, and only two observers are required.